Hi, all
I thought out a new way to resolve this problem: use CAS instead of refcount.
It not only resolve this problem, but also fix another issue,
can be expanded to support frontswap get_and_free mode easily.
And I use it in an zswap variant which writeback zbud page directly.
If it is accepted, I will resent it instead of the previous refactor patch
please see below, Request For Comments, Thanks.
On Fri, Oct 11, 2013 at 3:13 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Thu, Sep 26, 2013 at 11:42:17AM +0800, Weijie Yang wrote:
> > On Tue, Sep 24, 2013 at 9:03 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > > On Mon, Sep 23, 2013 at 04:21:49PM +0800, Weijie Yang wrote:
> > > >
> > > > Modify:
> > > > - check the refcount in fail path, free memory if it is not referenced.
> > >
> > > Hmm, I don't like this because zswap refcount routine is already mess for me.
> > > I'm not sure why it was designed from the beginning. I hope we should fix it first.
> > >
> > > 1. zswap_rb_serach could include zswap_entry_get semantic if it founds a entry from
> > > the tree. Of course, we should ranme it as find_get_zswap_entry like find_get_page.
> > > 2. zswap_entry_put could hide resource free function like zswap_free_entry so that
> > > all of caller can use it easily following pattern.
> > >
> > > find_get_zswap_entry
> > > ...
> > > ...
> > > zswap_entry_put
> > >
> > > Of course, zswap_entry_put have to check the entry is in the tree or not
> > > so if someone already removes it from the tree, it should avoid double remove.
> > >
> > > One of the concern I can think is that approach extends critical section
> > > but I think it would be no problem because more bottleneck would be [de]compress
> > > functions. If it were really problem, we can mitigate a problem with moving
> > > unnecessary functions out of zswap_free_entry because it seem to be rather
> > > over-enginnering.
> >
> > I refactor the zswap refcount routine according to Minchan's idea.
> > Here is the new patch, Any suggestion is welcomed.
> >
> > To Seth and Bob, would you please review it again?
>
> Yeah, Seth, Bob. You guys are right persons to review this because this
> scheme was suggested by me who is biased so it couldn't be a fair. ;-)
> But anyway, I will review code itself.
Consider the following scenario:
thread 0: reclaim entry x(get refcount, but not call zswap_get_swap_cache_page)
thread 1: call zswap_frontswap_invalidate_page to invalidate entry x.
finished, entry x and its zbud is not freed as its refcount != 0
now, the swap_map[x] = 0
thread 0: now call zswap_get_swap_cache_page
swapcache_prepare return -ENOENT because entry x is not used any more
zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM
zswap_writeback_entry do nothing except put refcount
Now, the memory of zswap_entry x and its zpage leak.
Consider the another scenario:
zswap entry x with offset A is already stored in zswap backend.
thread 0: reclaim entry x(get refcount, but not call zswap_get_swap_cache_page)
thread 1: store new page with the same offset A, alloc a new zswap entry y.
This is a duplicate store and finished.
shrink_page_list() finish, now the new page is not in swap_cache
thread 0: zswap_get_swap_cache_page get called.
old page data is added to swap_cache
Now, swap cache has old data rather than new data for offset A.
Error will happen If do_swap_page() get page from swap_cache.
Modify:
- re-design the zswap_entry concurrent protection by using a CAS-access flag
instead of the refcount get/put semantic
- use ZSWAP_SWAPCACHE_FAIL instead of ZSWAP_SWAPCACHE_NOMEM as the fail path
can be not only caused by nomem but also by invalidate.
Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
---
mm/zswap.c | 177 +++++++++++++++++++++++++++---------------------------------
1 file changed, 80 insertions(+), 97 deletions(-)
mode change 100644 => 100755 mm/zswap.c
diff --git a/mm/zswap.c b/mm/zswap.c
old mode 100644
new mode 100755
index cbd9578..fcaaecb
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -158,21 +158,23 @@ static void zswap_comp_exit(void)
* page within zswap.
*
* rbnode - links the entry into red-black tree for the appropriate swap type
- * refcount - the number of outstanding reference to the entry. This is needed
- * to protect against premature freeing of the entry by code
- * concurent calls to load, invalidate, and writeback. The lock
- * for the zswap_tree structure that contains the entry must
- * be held while changing the refcount. Since the lock must
- * be held, there is no reason to also make refcount atomic.
* offset - the swap offset for the entry. Index into the red-black tree.
+ * pos - the position or status of this entry. see below macro definition
+ * change it by CAS, we hold this entry on success or retry if fail
+ * protect against concurrent access by load, invalidate or writeback
+ * even though the probability of concurrent access is very small
* handle - zsmalloc allocation handle that stores the compressed page data
* length - the length in bytes of the compressed page data. Needed during
* decompression
*/
+#define POS_POOL 1 /* stay in pool still */
+#define POS_LOAD 2 /* is loading */
+#define POS_RECLAIM 3 /* is reclaiming */
+#define POS_FLUSH 4 /* is invalidating */
struct zswap_entry {
struct rb_node rbnode;
pgoff_t offset;
- int refcount;
+ atomic_t pos;
unsigned int length;
unsigned long handle;
};
@@ -216,7 +218,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
entry = kmem_cache_alloc(zswap_entry_cache, gfp);
if (!entry)
return NULL;
- entry->refcount = 1;
return entry;
}
@@ -225,18 +226,6 @@ static void zswap_entry_cache_free(struct zswap_entry *entry)
kmem_cache_free(zswap_entry_cache, entry);
}
-/* caller must hold the tree lock */
-static void zswap_entry_get(struct zswap_entry *entry)
-{
- entry->refcount++;
-}
-
-/* caller must hold the tree lock */
-static int zswap_entry_put(struct zswap_entry *entry)
-{
- entry->refcount--;
- return entry->refcount;
-}
/*********************************
* rbtree functions
@@ -380,6 +369,23 @@ static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
zswap_pool_pages = zbud_get_pool_size(tree->pool);
}
+/* caller must hold the tree lock, entry is on the tree
+* seldom called fortunately
+* return 0: free this entry successfully
+* return < 0: fail
+*/
+static int zswap_invalidate_entry(struct zswap_tree *tree, struct zswap_entry *entry)
+{
+ if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_FLUSH) == POS_POOL) {
+ rb_erase(&entry->rbnode, &tree->rbroot);
+ zswap_free_entry(tree, entry);
+ return 0;
+ }
+
+ /* encounter reclaim called by another thread */
+ return -1;
+}
+
/*********************************
* writeback code
**********************************/
@@ -387,7 +393,7 @@ static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
enum zswap_get_swap_ret {
ZSWAP_SWAPCACHE_NEW,
ZSWAP_SWAPCACHE_EXIST,
- ZSWAP_SWAPCACHE_NOMEM
+ ZSWAP_SWAPCACHE_FAIL,
};
/*
@@ -401,9 +407,10 @@ enum zswap_get_swap_ret {
* added to the swap cache, and returned in retpage.
*
* If success, the swap cache page is returned in retpage
- * Returns 0 if page was already in the swap cache, page is not locked
- * Returns 1 if the new page needs to be populated, page is locked
- * Returns <0 on error
+ * Returns ZSWAP_SWAPCACHE_EXIST if page was already in the swap cache
+ * Returns ZSWAP_SWAPCACHE_NEW if the new page needs to be populated,
+ * the new page is added to swap cache and locked
+ * Returns ZSWAP_SWAPCACHE_FAIL on error
*/
static int zswap_get_swap_cache_page(swp_entry_t entry,
struct page **retpage)
@@ -475,7 +482,7 @@ static int zswap_get_swap_cache_page(swp_entry_t entry,
if (new_page)
page_cache_release(new_page);
if (!found_page)
- return ZSWAP_SWAPCACHE_NOMEM;
+ return ZSWAP_SWAPCACHE_FAIL;
*retpage = found_page;
return ZSWAP_SWAPCACHE_EXIST;
}
@@ -502,7 +509,7 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
struct page *page;
u8 *src, *dst;
unsigned int dlen;
- int ret, refcount;
+ int ret;
struct writeback_control wbc = {
.sync_mode = WB_SYNC_NONE,
};
@@ -523,17 +530,23 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
spin_unlock(&tree->lock);
return 0;
}
- zswap_entry_get(entry);
+ /* maybe encounter load or invalidate called by another thread
+ * hold or give up this entry
+ */
+ if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_RECLAIM) != POS_POOL) {
+ spin_unlock(&tree->lock);
+ return -EAGAIN;
+ }
spin_unlock(&tree->lock);
BUG_ON(offset != entry->offset);
/* try to allocate swap cache page */
switch (zswap_get_swap_cache_page(swpentry, &page)) {
- case ZSWAP_SWAPCACHE_NOMEM: /* no memory */
+ case ZSWAP_SWAPCACHE_FAIL:
ret = -ENOMEM;
goto fail;
- case ZSWAP_SWAPCACHE_EXIST: /* page is unlocked */
+ case ZSWAP_SWAPCACHE_EXIST:
/* page is already in the swap cache, ignore for now */
page_cache_release(page);
ret = -EEXIST;
@@ -561,38 +574,23 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
page_cache_release(page);
zswap_written_back_pages++;
+ /* once we hold this entry and get here, we can free entry safely
+ * during the period which we hold this entry:
+ * 1. if a thread call do_swap_page concurrently
+ * we will get ZSWAP_SWAPCACHE_EXIST returned or
+ * do_swap_page will hit page we added in the swap cache
+ * 2. if a thread call invalidate concurrently
+ * invalidate thread should wait until this function end
+ */
spin_lock(&tree->lock);
-
- /* drop local reference */
- zswap_entry_put(entry);
- /* drop the initial reference from entry creation */
- refcount = zswap_entry_put(entry);
-
- /*
- * There are three possible values for refcount here:
- * (1) refcount is 1, load is in progress, unlink from rbtree,
- * load will free
- * (2) refcount is 0, (normal case) entry is valid,
- * remove from rbtree and free entry
- * (3) refcount is -1, invalidate happened during writeback,
- * free entry
- */
- if (refcount >= 0) {
- /* no invalidate yet, remove from rbtree */
- rb_erase(&entry->rbnode, &tree->rbroot);
- }
+ rb_erase(&entry->rbnode, &tree->rbroot);
spin_unlock(&tree->lock);
- if (refcount <= 0) {
- /* free the entry */
- zswap_free_entry(tree, entry);
- return 0;
- }
- return -EAGAIN;
+ zswap_free_entry(tree, entry);
+
+ return 0;
fail:
- spin_lock(&tree->lock);
- zswap_entry_put(entry);
- spin_unlock(&tree->lock);
+ BUG_ON(atomic_cmpxchg(&entry->pos, POS_RECLAIM, POS_POOL) != POS_RECLAIM);
return ret;
}
@@ -668,19 +666,15 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
entry->offset = offset;
entry->handle = handle;
entry->length = dlen;
+ atomic_set(&entry->pos, POS_POOL);
/* map */
spin_lock(&tree->lock);
do {
ret = zswap_rb_insert(&tree->rbroot, entry, &dupentry);
- if (ret == -EEXIST) {
- zswap_duplicate_entry++;
- /* remove from rbtree */
- rb_erase(&dupentry->rbnode, &tree->rbroot);
- if (!zswap_entry_put(dupentry)) {
- /* free */
- zswap_free_entry(tree, dupentry);
- }
+ if (unlikely(ret == -EEXIST)) {
+ if (zswap_invalidate_entry(tree, dupentry) == 0)
+ zswap_duplicate_entry++;
}
} while (ret == -EEXIST);
spin_unlock(&tree->lock);
@@ -709,9 +703,10 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
struct zswap_entry *entry;
u8 *src, *dst;
unsigned int dlen;
- int refcount, ret;
+ int ret;
/* find */
+find:
spin_lock(&tree->lock);
entry = zswap_rb_search(&tree->rbroot, offset);
if (!entry) {
@@ -719,7 +714,14 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
spin_unlock(&tree->lock);
return -1;
}
- zswap_entry_get(entry);
+ /* encounter entry reclaimed by another thread
+ * just retry until that thread get ZSWAP_SWAPCACHE_EXIST returned
+ * as we hold the entry, we can do anything: get or get_and_free
+ */
+ if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_LOAD) != POS_POOL) {
+ spin_unlock(&tree->lock);
+ goto find;
+ }
spin_unlock(&tree->lock);
/* decompress */
@@ -733,22 +735,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
zbud_unmap(tree->pool, entry->handle);
BUG_ON(ret);
- spin_lock(&tree->lock);
- refcount = zswap_entry_put(entry);
- if (likely(refcount)) {
- spin_unlock(&tree->lock);
- return 0;
- }
- spin_unlock(&tree->lock);
-
- /*
- * We don't have to unlink from the rbtree because
- * zswap_writeback_entry() or zswap_frontswap_invalidate page()
- * has already done this for us if we are the last reference.
- */
- /* free */
-
- zswap_free_entry(tree, entry);
+ BUG_ON(atomic_cmpxchg(&entry->pos, POS_LOAD, POS_POOL) != POS_LOAD);
return 0;
}
@@ -758,9 +745,9 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
{
struct zswap_tree *tree = zswap_trees[type];
struct zswap_entry *entry;
- int refcount;
/* find */
+find:
spin_lock(&tree->lock);
entry = zswap_rb_search(&tree->rbroot, offset);
if (!entry) {
@@ -769,19 +756,18 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
return;
}
+ /* encounter entry reclaimed by another thread
+ * just retry until that reclaim end.
+ */
+ if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_FLUSH) != POS_POOL) {
+ spin_unlock(&tree->lock);
+ goto find;
+ }
+
/* remove from rbtree */
rb_erase(&entry->rbnode, &tree->rbroot);
-
- /* drop the initial reference from entry creation */
- refcount = zswap_entry_put(entry);
-
spin_unlock(&tree->lock);
- if (refcount) {
- /* writeback in progress, writeback will free */
- return;
- }
-
/* free */
zswap_free_entry(tree, entry);
}
@@ -809,10 +795,7 @@ static void zswap_frontswap_invalidate_area(unsigned type)
*/
while ((node = rb_first(&tree->rbroot))) {
entry = rb_entry(node, struct zswap_entry, rbnode);
- rb_erase(&entry->rbnode, &tree->rbroot);
- zbud_free(tree->pool, entry->handle);
- zswap_entry_cache_free(entry);
- atomic_dec(&zswap_stored_pages);
+ zswap_invalidate_entry(tree, entry);
}
tree->rbroot = RB_ROOT;
spin_unlock(&tree->lock);
--
1.7.10.4
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